How do I add text comments to a blender file?
In order to access the text editor you would have to drag the window down. Try to change to the text editor from the 3d-view. 

As the Info Area is basically just the header, when you switch to text editor you will just see the editor header. Try pulling the header down from the interface to the 3d-view and you will see the text input space appear. (note recommended).

Instead maybe switch the timeline area (usually bottom left (clock item)) to text editor and put something in there. 

I recommend reading that.

https://www.blender.org/manual/interface/introduction.html